The present invention relates to control valves and pertains particularly to an implement control valve having a pressure controlled spool for obtaining precisely controlled variable torque from hydraulic motors.
Hydraulically operated implements and machines increasingly use higher rates of hyraulic flow and pressures. Such systems frequently use pressures on the order of 3000 psi to 5000 psi and volumes on the order of 50-100 gallons per minute of flow. Such high flows and pressures can result in very abrupt changes in pressure to the motor and high acceleration of the implements associated with such machines due to the high torque induced in the motor. This is especially a problem with rotary hydraulic motors wherein pilot control means is used for shifting the main control valves. Such high torque and the changes therein can cause starting and stopping as well as running away of the motor which can damage the machine as well as be uncomfortable for an operator.
Pilot operation of control valves for operation of hydraulic implements is normally employed in the prior art for remote operation as well as to reduce the effort required in movement of the controls to thereby reduce operator fatigue. Such prior art systems, however, normally have the disadvantage of a lack of precise control over positioning in relation to motor load or torque of the main control valve. The main control valve spool in such prior art systems is normally provided with rather stiff mechanical centering spring means in order to overcome the forces acting on the valve in order to shift the valve to a non-activating position. Thus, higher pressure pilot systems are required in order to shift the valves in the high pressure systems.
The centering spring pressure in such systems also tends to vary with displacement of the spool. This results in a non-linear relationship between the position of the control lever and the main control valve.
This prior art arrangement may also result in a system wherein the spool shifts quite abruptly from one position to another in view of the high forces required for such shifting. Such abrupt shifting of the control spools frequently causes rapid high acceleration of the motor from its starting position to operating speed and likewise a rapid deceleration of the motor from its operating to stopped position.
U.S. application Ser. No. 431,360 filed Jan. 7, 1974, now abandoned and assigned to the assignee hereof, is directed to a system for compensating for high pressures to control abrupt starting and stopping of a motor. However, that system fails to precisely control motor torque in direct relation to control lever position.
Attempts heretofore known to provide pilot operated control systems capable of more closely controlling the main control spool has resulted in very complex and expensive systems.